Formation from Mesionic Oxazoles

Formation from Mesionic Oxazoles. In the course of their comprehensive researches on 1,3-dicycloadditions, Huisgen and his group have developed 

Hashimoto, and M. Ohta, Nippon Kagaku Zasshi, 1957, 78, 707. 

The reaction is performed most advantageously by generating the required starting material (150) in situ thus a stirred mixture of iV-methyl-JV-benzoylphenylglycine, acetic anhydride, and carbon disulphide evolves carbon dioxide, affording (154) in 96% yield. The procedure is particularly useful when the oxazolium 5-olate is not isolable.  

The deep-orange 3-methyl-2,4-diphenylthiazolium 5-thiolate (154) forms yellow solutions in acids, presumably owing to protonation of the exocyclic sulphur. Its oxidation by hydrogen peroxide in acetic-formic acids produces the 5-sulphonate (155) (91%) subsequent treatment with concentrated hydrochloric acid yields 2,4-diphenylthiazole (156). Anhydro-5-hydroxy-3-methyl-2-phenylthiazolium hydroxide (157) resists alkylation by conventional reagents, but is readily 0-alkylated by triethyl-oxonium fluoroborate, yielding 5-ethoxy-3-methyl-2-phenylthiazolium fluoroborate (158). In this respect, it resembles related mesionic systems based on 1,3,4- and 1,2,3-thiadiazoles (c/. Chapter 15). 

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